In general, shot peening is an effective surface treatment method, in which particles referred to as a projecting material (or also referred to as “shot”, “shot material”, “medium”, “abrasive material”, or the like) are projected onto the surface of a material to be treated, compressive residual stress is applied, and fatigue strength can be improved, and is also applied to automobile components such as springs and gears, metal mold materials, or the like. As in the case of, e.g., gears subjected to carburizing and quenching treatment, the higher hardness of materials to be treated has been achieved, and also the higher hardness of projecting materials for these materials has been demanded. In other words, high compressive residual stress cannot be obtained by performing a shot peening in which a low-hardness projecting material is used for a high-surface-hardness material to be treated. Moreover, with the further need for reduction in the weight of an automobile component or the like, it is necessary to perform shot peening of a material to be treated which has increasingly high hardness and, therefore, there is a demand for a projecting material having further high hardness.
On the other hand, not only a projecting material having an average particle diameter of around 500 to 1000 μm used for standard shot peening, but also a projecting material having an average particle diameter of around 100 μm is used for fine-particle shot peening. The fine-particle shot peening does not excessively roughen the surface of a material to be projected, but allows large compressive residual stress to be applied to a portion closer to the treated surface, and greater improvement in fatigue strength than that in the case of standard shot peening is therefore expected. In recent years, use of a projecting material having a further small particle diameter has also been examined to make further use of the characteristics of the fine-particle shot peening.
The inventors have proposed, in Japanese Patent Laid-Open Publication No. 2007-84858 (Patent Literature 1), a projecting material that comprises a Fe2B-based boride and an iron-base solid solution of BCC and/or FCC and contains 5 to 8% of B as an inexpensive projecting material with high hardness. One of the characteristics of the projecting material is in that the addition of 5% or more of B results in generation of a large amount of high-hardness Fe2B, thereby increasing the hardness of the whole particles.